Linux: bypass: consolidate copy_page macros into a single function

Linux: bypass: consolidate copy_page macros into a single function

The copy_page(s) macros are very similar; combine them into a
single function that can be used for all cases.

This will make it easier to add some pre-processor logic around
the kmap_atomic calls to adapt to Linux API changes.

Change-Id: I6daf7cb8ce49b743cc1b4532da91d9625a90d0ca
Reviewed-on: http://gerrit.openafs.org/7980
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>